System of control for electric spot welding



J. P. TARBOX March 17, 1936.

SYSTEM OF CONTROL FOR ELECTRIC SPOT WELDING Original Filed March 27, 1951 I N VEN TOR Patented Mar. 17, 1936 PATENT OFFICE SYSTEM OF CONTROL FOR ELECTRIC SPOT WELDDTG John P. Tarbox, Philadelphia, Pa... assignor to Edward G. Philadelphia, Pa., vania Application March 27, 1931, Serial No.

Budd Manufacturing Company,

a corporation of Pennsyl- Renewed September 21, 1933 24 Claims.

They are moreover particularly adapted for extremely instantaneous indirect welding, the term indirect here being applied to those welds in which action and reaction to the electrodes cannot by reason of the construction of the work be 10 absorbed fully within the welding apparatus as well as those weldsiii which for similar reasons the electrodes may not directly oppose each other on opposite faces of the work.

One object is the achievement of control 15 through continuously rotating contact devices whereby mechanical and electrical inertia-s are wholly eliminated from influence upon the accuracy with which the extremely instantaneous time intervals are measured. Scarcely second in 20 importance is the object of achieving indirect welding in instantaneous time intervals.

Viewed from the opposite side it may be said that one object of the invention is the elimination of the influence of mechanical and electrical inertia upon the accuracy of measurement of instantaneous time intervals for spot welds. From this angle the invention is attained by the employment of continuously rotating welding circuit control contacts. More 3 specifically still it is attained by the employment of a pair of relatively high and low speed commutators having substantially commensurately proportioned welding circuit control contacts operating in series with each other and 3:, which commutators are geared together commensurately, and control enabling means for these commutators in the form of electro-magnetic switching devices manually or automatically governed through the low speed commutator 40 and having but a single effective enabling operation per complete revolution of the low speed commutator.

One of the objects of the invention, the achievement of indirect instantaneous spot welding, is

4., attained by utilizing Welding electrodes in the manner of an anvil and hammer, the hammer blow of which develops the maximum welding pressure, and so co-ordinating or simulating synchronism between the harm. er blow of the one electrode toward the other and the instantaneous application of welding current that the contact surfaces to be Welded together are fused under those initial and final pressures, the latter of which are developed particularly by the hammer I blow which will secure an eflicient instantaneous measured free weld. Such co-ordination and simulated synchronism are attained through control of the hammer blow mechanism by the same continuously rotating timing apparatus through which the extreme instantaneous time periods are of mechanical and electrical inertia.

The accompanying drawing discloses my invention diagrammatically, the single figure being the principal and complete diagrammatic illustration of the system and apparatus.

The welding circuits are derived from a transformer l0 having a primary H and a secondary i2. Connected with the opposite terminals of the secondary 12 are an anvil type electrode I3 and a hammer type electrode 54. The anvil type i3 is adapted for holding by an operator interiorly of the work I5 as, for example, interiorly of the hull of a fiying boat to be made of metal, or interiorly of the body of another vessel. It is characteristic of such bodies that their outer covering is of such expansive and all inclusive nature that it becomes impracticable to eflect mechanical interconnection for direct reaction between the inner electrode l3 and the outer situations exist where the one electrode I3 is required to make connection at a point remote from the axis of the other [4.

The electrode I4 is the more conventional type, rod-like in form and adapted through an extension shank 6 and hand-grip I! for direct manual application to spots on the exterior of the work at which it is desired to weld. The constructions and forms and even the functions of these parts may be varied to considerable extent in accordance with the variations in the work and the degrees of initial pressure to be applied.

The primary II is energized from a source of power l8 connected with the transformer through a line switch I9 over a circuit designated generally 20 throughout the diagram. This circuit 20 is opened and closed by the instantaneous time switch apparatus, the principal elements of which are the relatively high speed commutator 2|, the relatively low speed commutator 22 and the welding circuit enabling apparatus designated generally 23. The welding circuit 20 includes fixed and adjustable contact brushes 24, 25 associated with contact 26 on the high speed commutator 2| connectible in series with brushes 21 and 28 cooperating respectively with segments 29 and 30 on the low speed commutator, by means of enabling circuit breaker contact 3|.

The high speed commutator 2| is driven by a variable speed electric motor 32 capable of very I4. Similar 25 high speed through the diagrannnatically illustrated shaft 33, the speed of the motor 32 being variable by a rheostat 34. The segment 26 carried by commutator 2|, the main body of which 35 is of insulating material, is of varying peripheral extent being peripherally long at one extremity and peripherally short at the other, tapering' substantially uniformly from one end to the other. Its one edge 36 is parallel to the axis of the commutator while its other edge 31 is oblique thereto. Segment 26 is permanently connected with the continuous peripheral conducting ring 38 to which fixed brush 24 continuously supplies the primary potential. The relatively movable brush 25 is movable along an axially extending screw 39 axially fixed in position, but rotatable to axially adjust brush 25 by a crank 40 or equivalent means whereby the brush 25 may be adjusted axially of the commutator 2| to co-a'ct' with any axial portion of segment 26 and thereby vary the peripheral extent of contact between itself and segment 26. The time of closure of circuit 20 is by this means adjustable through a wide range of instantaneous time intervals. With the motor 32 capable of say 3600 R. P. M. and having a speed variable to any desired extent through rheostat 34 or its equivalent, the time of closure of the primary circuit at high speed rotary contacts 24, 25, 26 may be varied with extreme fineness over an extremely wide range of extremely high speeds.

Relatively low speed commutator 22 is com- 'mensurately geared to shaft 33 through reduction gearing 4| connected with a shaft 42 on which commutator 22 is mounted. This commutator bears four rotary contacts, the one 29 already mentioned as inthe welding circuit 20 and angularly continuous, another 30 likewise in the welding circuit 20 having an are slightly greater than the value commensurate with the greatest arc of segment 26 short of a complete circuit, greater specifically by a peripheral extent designated a, a third contact 43 extending over the entire periphery of the commutator.22 excepting through the major portion of the angle of extent of the segment 30, and a fourth 44 of an extent generally similar in angular location and degree to that of the segment 30. All four of these segments are commonly connected together through the common connecting conductor 45. More specifically the ratio between the arc of angular extent b of segment 36 and the arc of maximum extent 0 of segment 26 on commutator .2| is substantially equal to the ratio'of the reduction of gears 4|.

Further, commutators 2| and 22 are commensurately geared together and the gears 4| in such mesh that whenever brushes 25 and 28 contact respectively with the arcs b and c of the segments 36' and 26, the makingof such contact and its breaking will occur substantially at the same time. On the part of contact 30, however, the arc b occupying this syn- 1 chronized and commensurate relation to the are c, is slightly added to at both ends whereby brush 28 makes contact slightly ahead of the brush 25 and segment 26 and breaks contact slightly after the brush 2,5 and segment 26.

The contact 3| of the enabling apparatus 23 is normally open, the electro-magnet 46 is provided for closing it and thereby enabling the welding circuit 20. The circuit of electro-ma net 46 is designated generally 41. It is made from apush button 48 which derives its potential from a brush 41a which coacts with the segment 43 of commutator 22. Circuit 41 so made extends through the normally closed contact 430, of a relay 49 of the enabling apparatus 23, a function of which is to open and hold open circuit 4'! at contact 48a so long as push button 48 is held down. Relay 49 is initially energized over circuit 56 from a make contact 5| on the electro-magnet '46. Being energized it locks itself up over circuit 52 made by its own locking contact 53 and extending through a normally open back contact 54 associated with the push button 48. But simultaneously with the energization of its lockout relay 49 enabling electro-magnet 46 closes two locking circuits, the one 55 from its contact 56 by way of circuit 41 to segment 43 and the other 51 by its contact 58 to the circuit 26 emanating from segment 36 and constituting a part of the primary welding control circuit 26.

The segment 44 controls through an angularly adjustable brush 59 the circuit 60 of an electroa magnetic hammer actuator 6| associated with the hammer type electrode I4. A coil 6| of this The operation is as follows: To spot weld the work l5, the system is closed to the source of power l8 at switch I9. Crank 40 is turned to rotate screw 39 and adjust brush 25 axially to de- I fine the time adjudged suitable for the work in hand. Transformer voltage regulator 64 is adjusted to vary the turns ratio of the transformer and therefore the voltage of the secondary l2 'for the current application desired. Through the switch 65 motor 32 of. the commutators 2|, 22 is started. Its revolutions are adjusted to the value properly correlated with the time as set by the brush 25, let us now say, to full speed. Thereupon the electrodes l3 and I4 are adjusted manually to opposite sides of the work, one from the interior and the other from the exterior and appropriate initial pressure is applied through bearing on the hand-grip I! manually or otherwise. Thereupon push button 48 is closed with the following sequence of operation. Circuit 41 is closed at contact 48a. from a source of power l8 via that branch of circuit 20 connected with contact 21 and segment 29 of commutator 22, interconnecting conductor 45, segment 43 when or as soon as it contacts with brush 41a, contact 48, contact 48a, electro-magnet 46 and by way of conductor 66 back to the source. Electro-magnet 46 pulls up and then closes up the following circuits: It closes circuit 56 of control lockout relay 49 at its contact 5| from conductor 26 connected with one side of the source through the coil of relay 49 and by a conductor 66 to the other side of the source. The pulling up of relay 49 opens the energizing circuit 41 of electromagnet 46. However, electro-magnet 46 has simultaneously with its closure of circuit 50 of relay 49 closed its locking contacts 56 and 58 thereby first locking itself up over circuit 55 to segment 43 and second parallelly locking itself up over circuit 51 to circuit 20 at its connection with brush 28 and segment 30. Both of these mary H of transformer I!) are energized to the ends of passing welding current between electrodes l3 and I4 through the work l5, and striking a hammer blow of electrode i4 upon the work through the impact of the core 62 of the actuator 64 upon it. The relations and timings of the parts are such that welding current and the hammer blow may occur precisely at the proper relative moment.

- When the weld has been thus made segment '30 passes from beneath brush 28 and the locking circuit 57 of electro-magnet 46 is broken, opening circuit 39 at contact 3|. Inasmuch as the initial energizing circuit 4'! is open at contact 48a as long as relay 49 is energized, the circuit 20 is disabled the moment segment 36 passes from beneath brush 2B and may not again be enabled until manual control contact 48 is released and unlocks relay 49 at its back contact 54. After this is done the contact 48 may again be made for a repeat operation. No matter how many revolutions commutator 2! may make or how many commutator 22 may make, but one weld can be made per actuation of the main control 48 or per actuation of electro-magnet 46 and its lockout relay 49, therefore but one spot weld may be made per operation of the welding circuit enabling apparatus 23.

The relative angles covered by segments 43 and 30 are such that brush 47a does not contact with segment 43 during the principal portion of the dwell of brush 28 on segment 36. Thereby it is impossible to energize electro-magnet 46 during the simultaneous contact of brushes 25 and 28 respectively with segments 26 and 36. This means that a weld cannot be initiated in such relation to the primary high speed timing contacts 25, 26 as to foreshorten the adjusted period. However, as has been noted segment 36 is extended beyond the arc b commensurate and synchronized with the are c of segment 26 by an angle a. and furthermore to overlap by an are d the trailing end of segment 43. The overlapping angle (1 insures efiective transfer of the lockup of electro-magnet 46 from the initial locking circuit H to the welding circuit lockup The angle a over the welding are I) insures adequate time for electro-magnet 46 to operate fully before the crush 28 commences to traverse the commensurate and synchronized are I), which travel is precisely within the maximum time of the weld as determined by the arc 0. At the leaving end of the segment 33 as respects brush 28 there is an angle 2 of commutator revolution in which neither brush 28 nor brush 47a contacts with a segment. This allows for a complete break of circuit 2!! at the leaving end of contact 36 and a complete restoration of electro-magnet 46 before brush 41a engages again with segment 43 whereby a weld ing period once made may not be unduly prospeed timing contact 26. Indeed the initial opening of circuit 20 may be altogether achieved at the rectilinear axial leaving edge 36 of the highspeed contact 31, or by this edge 36 in conjunction with the leaving end of segment 36. If the conjunctive action be not desired, then the leaving end of segment 38 may be extended somewhat beyond its commensurate position as defined by the arc b and the idle are e extended accordingly. When times are adjusted by movement of brush 25 to be measured by arcs of segment 26 less than the full are 0, brush 25 makes contact with segment 26 behind the commensurate position of brush 28 at the inception of are I), but this is of no moment since it but prolongs the period defined by arc a in which the elcctro-' magnet 46 and its lock-out 49 may come fully into action.

The segment 44 while generally co-extensive with the segment 30 may have its ends terminae one side or the other of the ends of segment 36. The important thing is that brush 59 is angularly adjustable whereby to accurately time the rela tion of closure of circuit 66 of the hammer blow actuator 6| as respects the closure of the welding circuit 20 at the ultimate control contacts 25, 23. This adjustment is eflected by the mechanism 68, shown in diagram as a screw and -nut mechanism, and the extent of segment 44 and the extent of movement of the brush 59 is intended to be such that the adjustment will bring about a most efiicient timing between the hammer blow of electrode 44 and the application of the welding current over the circuit 26.-

Summing up, it will be seen that there has been achieved a method and system by means of which indirect instantaneous welding may be done, by means of which the pressures for such welding may be derived by impact and sustained after the impact with the degree determined by the relative inertias of the parts of the welding apparatus, for the inertias of the anvil l3 and of the hammer blow apparatus is such that the necessary pressures may be attained and sustained for the necessary extremely short periods of the instantaneous weld. Nevertheless the timing of both the welding pressure and the welding current are determined with utmost accuracy without influence from either mechanical or electrical inertia and without lost motions, for there is continuous rotation of the commutators 2|, 22 and the time period through the adjustment of brush 25 when entered upon is free from encroachment from any *of these factors. The breaking of the welding circuit is at extremely high speed, and snappy and sharp, as brush passes ed the edge 3'11 of segment 26. The time intervals measured may be extremely fine, even so small as from 1/1000 to l/10,000 seconds and of course in any value of longer in ervals. Yet

the operation of the manually controlled and enabling apparatus through the low speed commutator not only gives ample time for manual manipulation and for full and electro-magnetic response of the relays 46 and 46 without false operations, but also constitutes an efiective unit spot controlunder which but one ,weld can be made for one manual manipulation of the manually controlled contact 48.

The features of my inventive concept herein explained but not herein claimed, comprise subject matter of a copending application.

I contemplate the use of all equivalent electrical means in carrying out the generic spirit of my invention and particularly do I contemplate 'to me.

utilizing each and all electric circuit and contact expedients which may be had in attaining and maintaining in good order the electrical and mechanical action and reaction.

The terminology I have used is the best now known to me, even as the embodiment of the invention I have disclosed is the best now known In order to protect those other embodiments to which the development of the art unavoidably extends the generic spirit of my invention, and their modifications likewise incorporating the generic spirit, the terminology should be given commensurate breadth independent of the present circumstances which determine it.

What I claim as new and useful and desire to protect by Letters Patent is:

l. A timing system for electric circuits com.- prising a continuously rotating circuit closing switch having an adjustable period of circuit closure, said switch comprising a commutator, the movable and fixed parts of which are in continuous engagement, an enabling device to out said switch into and out of circuit closing and opening action, and means limiting the enabling action of said enabling device to a single circuit closing operation of said continuously rotating switch for each single operation of said enabling device.

2. A timing system for electric circuits comprising a high speed continuously rotating circuit control switch operating to control a circuit through continuously rotating contacts, said switch comprising a commutator, the movable and fixed parts of which are in continuous engagement, an enabling device for cutting into and out of circuit closing and opening action, said high speed continuously rotating switching apparatus and having control of the efiectiveness of said switching apparatus, and means limiting the enablem'ent of said switching apparatus by said enabling device to a single efiective control of the circuit through its continuously rotating contacts for a single operation of the enabling device.

3. A timing system for electric welding circuits comprising a switching apparatus embodying high speed and relatively low speed continuously rotating welding circuit closing contacts commensurately driven together and proportioned in contact extent and proportioned commensurately to the relative speeds, said contacts being relatively movable and comprising commutators, the fixed and movable parts of which are in continuous engagement, and an auxiliary controlcircuit controlling the efiectiveness of said switching apparatus upon a welding circuit operating through the low speed switching contact.

4. A timing system for electric welding ma chines comprising continuously rotating high speed and low speed commutators commensurately geared together and having joint control over a welding circuit, which circuit passes through said commutators in series, said commutator having relatively .fixed and movable parts which are in continuous engagement, and an auxiliary control circuit for the commutators operating to exercise the control through the low speed cormnutator.

5. A timing system for electric welding machines comprising continuously rotating high speed and low speed commutators commensu- 'rately geared together and having joint control over a welding circuit, and control enabling means for the high speed commutator operating to exercise the control through the low speed commutator, which enabling means comprises a relay switch connecting the commutators in series, and a circuit controlling means for the relay adapted to close its circuit through the slow speed commutator and having a single effective circuit closing operation per cycle of its own operation.

6. A timing system for electric welding machines comprising continuously rotating high speed and low speed commutators commensurately geared together and having joint control over a welding circuit, a control enabling relay for cutting into and out of circuit closing and opening action the high speed commutator and operating to exercise the control through the low speed commutator, switching contacts on the low speed commutator and separate therefrom controlling the circuit of said relay in series and out of step with the welding circuit gcontrol contacts, and dual locking circuits for said relay, one, extending through the relay enabling contact of the commutator and the other through the welding circuit control contact thereof.

' 7. A timing system for electric welding machines ccmprising continuously rotating circuit controlling commutators, one rotating at ex.- tremely high speed and having contacts at which the welding circuit is broken, and the other commensurately geared together and in series therewith rotating at relatively low speed and controlling the completion of the welding circuit through the high speed commutator, said commutators having their fixed and movable parts in continuous engagement and a welding circuit including said series of high speed and low speed contacts.

8. A timing system for electric welding machines comprising continuously rotating circuit controlling commutators, one rotating at extremely high speed and having contacts at which the welding circuit is broken, and the other commensurately geared in series therewith rotating at relatively low speed and controlling the completion of the welding circuit through the high speed commutator together with means to vary the period of closure of the circuit, and a welding circuit including said high speed and low speed contacts.

i 9. A timing system for electric spot welding circuits comprising a switching apparatus embodying high speed and relatively low speed continuously rotating welding circuit energizing contacts, said contacts being commensurately driven and proportioned commensurately in contact extent and to the relative speeds, said contacts being relatively movable and comprising commutators having their fixed and movable parts in continuous engagement, and an auxiliary control circuit controlling the efiectiveness of said switching apparatus upon a welding circuit operating through the low speed switching contact, and means for adjustably varying the efiective period of closure of said high speed contacts within pre-determined maximum limits.

10. A timing system for electric spot welding circuits comprising a switching apparatus embodying high speed and relatively low speed continuously relatively rotating welding circuit energizing contacts proportioned in contact eX- tent and commensurately driven, said contacts being relatively movable and comprising commutators having their fixed and movable parts in continuous engagement, and an auxiliary control circuit controlling the effectiveness. of said switching apparatus upon a welding circuit operating through the low speed switching contact,

and means adjustable to provide variable speeds of said contacts without afiecting said synchronous relation therebetween.

11. A timing system for electric spot welding machines comprising a continuously rotating circuit closing switch having'an adjustable period of circuit closure, an enabling device to out said switch into and out of circuit closing and opening action, and means limitingthe enabling action of said enabling device to a single effective circuit closing operation of said continuously rotating switch for each single operation of said enabling device, said switch comprising a plurality of continuously driven high speed and low speed relatively movable switching elements synchronously relatively movable with respect to each other.

12. A timing system for electric welding circuits comprising a switching apparatus embodying high speed and relatively low speed continuously relatively rotating welding circuit energizing contacts proportioned in contact extent and commensurately driven, said contacts being relatively movable and comprising commutators having fixed and-movable parts in continuous engagement, reducing gearing interposed between said high and low speed contacts, and

an auxiliary control circuit controlling the effectiveness of said switching apparatus upon a welding circuit operating through the low speed switching contact.

v13. A timing system for electric welding,,circuits comprising a switching apparatus embodying high speed and relatively low speed continuously rotating welding circuit closing contacts proportioned in contact extent and commensurately driven, a circuit passing through said contacts in series relatiom'an auxiliary control circuit controlling the effectiveness of said switching apparatus upon a welding circuit operating through the low speed switching contact, and means whereby said low speed contacts close and open a portion of said welding circuit respectively before and after the closure and opening of the high speed contacts in said circuit.

14. In an electric spot welding apparatus including a pair of electrodes arranged to be brought into contact with .work, an electric circuit\ for supplying the electrodes with energy periodically fluctuating to and from zero, the combination of means for operatively energizing said circuit and for automatically maintaining said circuit energized for a definite period of time and then effectively de-energizing said circuit, said means including high and low speed relatively movable contacts synchronously movable with respect to each othenmeans for initiating the operation of said means, and means for preventing more than 'a single energization of said circuit to welding strength for each actuation of said initiating means.

15. A timing system for electric welding machines comprising a continuously rotating cincuit closing switch having an adjustable period of circuitclosure, an auxiliary control circuit comprising a switching apparatus embodying high speed and relatively low speed continuously rotating welding circuit closing contacts proportioned in contact extent and commensurately driven, enabling means controlling the effectiveness of said switching apparatus upon a welding circuit operating through the,low speed switching contact, and electrical relay means operating by circuits through said circuit closing contacts and arranged to prevent operation of said enabling means at a time when the high speed contacts are in engagement.

17. A timing system for electric welding machines comprising continuously rotating high speed and low speed relatively movable contacts commensurately geared together and having joint control over a welding circuit, a control enabling means including a relay for cutting into and out of effective circuit closing and opening action said relatively movable contacts and operating to exercise the control through the low speed contacts, switching contacts synchronously operated in relation to the relatively movable contacts and arranged to control the operation of said enabling means in'series with and out of step with the welding circuit control contacts, and means arranged to provide dual locking circuits for said relay, one of said circuits extending through a relay enabling contact of said relatively movable contacts and the other through a welding circuit control contact of said relatively movable contacts.

18. A timing system for electric welding machines comprising a continuously rotating welding circuit control switch operating to control a welding circuit through a plurality of continuously rotating contacts continuously operated to make and break contact, an enabling device for cutting into and out of circuit closing and opening action said high speed continuously rotating switching apparatus and having control of the efiectiveness of said switching apparatus, and means operating through said continuously rotating contacts limiting the enablement of said switching apparatus and continuously, operated to make and break contact, a circuit arranged to be energized for an adjustable definite timed period through said contacts, an enabling device arranged to out said contacts into and out of effective circuit closing and opening action, means to initiate the operation of said enabling device, and means including a locking relay, jointly operating through said circuit timing contacts and through said initiating means, whereby to limit the enabling action of said enabling device to a single effective circuit closing operation of said circuit contacts for each single operation of the initiating means.

20. A timing system for electric spot welding machines comprising a continuously rotating circuit closing switch having an adjustable period of circuit closure, an enabling device to out said switch into and out of circuit closing and opening action, and means limiting the enabling action of said enabling device to a single effective circuit closing operation of said continuously rotating switch for each single operation of said enabling device, said switch comprising a plurality of continuously driven relatively high speed and low speed relatively movable switching elements synchronously relatively movable with respect to each other, said system arranged so that the circuit opening action always occurs at the high speed relatively movable contacts.

21. A timing system for electric spot welding machines comprising a continuously rotating circuit closing switch having an adjustable period of circuit closure, an enabling device to out said switch into and out of circuit closing and opening action, and means limiting the enabling action of said enabling device to a single effective circuit closing operation of said continuously rotating switch for each single operation of said enabling device, said switch comprising a plurality of continuously driven high speed and low speed relatively movable switching elements synchronously relatively movable with respect to each other, and means for adjusting the period of closure of said relatively movable switching elements, said system arranged so that the circuit opening action always occurs at the high speed relatively movable contacts.

22. A timing system for electric spot welding machines comprising a continuously rotating circuit closing switch having an adjustable period of circuit closure, an enabling device to cut said switch into and out of circuit closing and opening action, and means limiting the enabling action of said enabling device to a single efiective circuit closing operation of said continuously rotating switch for each single operation of said enabling device, said switch comprising a plurality of continuously driven high speed and low speed relatively movable switching elements synchronously relatively movable with respect to each other, and means arranged to prevent a foreshortening of said pre-set time period of circuit closure.

23. A timing system forelectric spot welding machines comprising a continuously rotating circuit closing switch having an adjustable period of circuit closure, an enabling device to out said switch into and out of circuit closing and opening action, and means limiting the enabling action of said enabling device to a single effective circuit closing operation of said continuously rotating switch for each single operation of said enabling device, said switch comprising a plurality of continuously driven high speed and low speed relatively movable switching elements synchronously relatively movable with respect to each other, and means arranged to prevent a foreshortening of said pre-set time period of circuit closure, said means including a lock out relay.

24. A timing system for electric spot welding machines comprising a continuously rotating circuit closing switch having an adjustable period of circuit closure, an enabling device to out said switch into and out of circuit closing and opening action, and means limiting the enabling action of said enabling device to a single eiiective circuit closing operation of said continuously rotating switch for each single operation of said enabling device, said switch comprising continuously driven relatively movable switching elements, relatively movable synchronously with respect to each other, and means operating through said relatively movable switching ele- 

